In the recent trend of digital technology in a digital video recorder and communication satellite broadcast, it is attempted to control devices using digital technology by connecting to one network.
In such a network, the IEEE1394 interface is expected to be used widely in the future as a digital interface excellent in universality owing to the following reasons.
1) Signals can be transmitted between arbitrary devices regardless of form of their connection.
2) Live wires of cables used in connection can be inserted and removed.
3) Both isochronous data, which is realtime data such as audio-video (AV) signals, and asynchronous data, which is non-realtime data such as commands for devices, can be transmitted simultaneously. It is hence easy to use for the personal computer (PC) and its related devices.
FIG. 36 shows an example of configuration of system operating by using the IEEE1394 interface.
As shown in FIG. 36, a set top box (STB) 600, a VHS digital video tape recorder unit (D-VHS) 700, and a television receiver (TV) 800 are connected to an IEEE1394 bus 900 having plural channels. These devices constitute one system controllable as a logical unit.
The STB 600 includes a tuner which is not shown in the drawing. The D-VHS 700 includes a video cassette recorder (VCR) and a tuner, neither shown. The TV 800 includes a monitor and a tuner, neither shown.
Herein, the tuner, VCR and monitor are controlled as subunits in each unit.
The unit and subunit have plugs for exchanging signal input and output.
In the unit, the following plugs are defined.
1) Plugs for input: a) digital input plug, b) external input plug, c) digital asynchronous input plug.
2) Plugs for output: a) digital output plug, b) external output plug, c) digital asynchronous output plug.
In the subunit, the following plugs are defined.
1) Plug for input: destination plug.
2) Plug for output: source plug.
When building a system connecting plural devices by using such IEEE1394 interface, it is important to set the connection state of plural units, including connection with the subunit in the unit, in particular, the signal source to the intended plug, and to know the signal source.
For knowing or setting the connection state between subunits, as shown in FIG. 37, it is necessary to exchange “Connect control command” and “Connect status command.” These commands are defined in “AV/C Digital Interface Command Set General Specification, Version 3.0. TA document number 1998003.” The commands includes a status command for inquiring a plug connected to a designated plug, a control command for connecting two designated plugs, and others.
In particular, in the case that multiple subunits are present on a signal path between the intended plug and the output source of the signal, if the device is controlled by using these commands, there are too many commands to be exchanged. Therefore, many steps are needed in order to control the system.
Further, when making protected connection of signals between two specific units, the point-to-point connection is employed. Hitherto, in the IEEE1394, it is designed to cut off the connection only in the unit established in connection.